Address format

MCTP addresses (also referred to as EIDs, or Endpoint Identifiers) are single-byte values, typed as mctp_eid_t. Packets between a local and remote endpoint are identified by the source and destination EIDs, plus a three-bit tag value.

Addressing data is passed in socket system calls through a sockaddr_mctp structure, defined as:

typedef uint8_t        mctp_eid_t;
struct mctp_addr {
    mctp_eid_t         s_addr;
};
struct sockaddr_mctp {
    unsigned short     smctp_family;  /* = AF_MCTP */
    uint16_t           __smctp_pad0;
    int                smctp_network; /* local network identifier */
    struct mctp_addr   smctp_addr;    /* EID */
    uint8_t            smctp_type;    /* message type byte */
    uint8_t            smctp_tag;     /* tag value & owner */
    uint8_t            __smctp_pad1;
};

Sending messages

Messages can be transmitted using the sendto(2) and sendmsg(2) system calls, by providing a sockaddr_mctp structure describing the addressing parameters.

ssize_t               n;
const char            *buf;
struct sockaddr_mctp  addr;
/* unused fields must be zero */
memset(&addr, 0, sizeof(addr));
/* set message destination */
addr.smctp_family = AF_MCTP;
addr.smctp_network = 0;
addr.smctp_addr.s_addr = 8; /* remote EID */
addr.smctp_tag = MCTP_TAG_OWNER;
addr.smctp_type = MYPROGRAM_MCTP_TYPE_ECHO; /* example type */
buf = "hello, world!"
n = sendto(sd, buf, 13, 0,
           (struct sockaddr *) &addr, sizeof(addr));

Here, the sender owns the message tag; so MCTP_TAG_OWNER is used as the tag data. The kernel will allocate a specific tag value for this message. If no tag is available, sendto(2) will return an error, with errno set to EBUSY. This allocated tag remains associated with the socket, so that any replies to the sent message will be received by the same socket.

Sending a MCTP message requires the CAP_NET_RAW capability.

Receiving messages

Messages can be received using the recvfrom(2) and recvmsg(2) system calls.

char                  buf[13];
socklen_t             addrlen;
struct sockaddr_mctp  addr;
addrlen = sizeof(addr);
n = recvfrom(sd, buf, sizeof(buf), 0,
             (struct sockaddr *) &addr, &addrlen);

In order to receive an incoming message, the receiver will need to either have previously sent a message to the same endpoint, or performed a bind(2) to receive all messages of a certain type:

struct sockaddr_mctp  addr;
addr.smctp_family = AF_MCTP;
addr.smctp_network = MCTP_NET_ANY;
addr.smctp_addr.s_addr = MCTP_ADDR_ANY;
addr.smctp_type = MYPROGRAM_MCTP_TYPE_ECHO; /* our 'echo' type */
int rc = bind(sd, (struct sockaddr *) &addr, sizeof(addr));

This call requires the CAP_NET_BIND_SERVICE capability, and will result in the socket receiving all messages sent to locally-assigned EIDs, for the specified message type.

After a recvfrom(2) or recvmsg(2) returns a success condition, the provided address argument will be populated with the sender's network and EID, as well as the tag value used for the message. Any reply to this message should pass the same address and tag value (with the TO bit cleared) to indicate that is is directed to the same remote endpoint.